Matrix displays are well known in the art, where pixels are illuminated using matrix addressing as illustrated in FIG. 1. A typical passive matrix organic electroluminescent display 100 comprises a plurality of picture or display elements (pixels) 160 that are arranged in rows and columns. The display incorporates a column data generator 110 and a row select generator 120. In practical operation, each row is sequentially activated via row line 130, where the corresponding pixels are activated using the corresponding column lines 140. In a passive matrix display, each row of pixels is illuminated sequentially one by one, whereas in an active matrix display, each row of pixels is first loaded with data sequentially. Namely, each row in the passive matrix display is only “active” for a fraction of the total frame time, whereas each row in the active matrix display can be set to be “active” for almost the entire total frame time.
A typical active matrix organic electroluminescent display has been achieved in the prior art (See U.S. Pat. No. 6,157,356). In this patent, as shown in FIG. 2, the emitting pixel includes a switch transistor M1230, a drive transistor 240, a data line 210, a select line 220, a storage capacitor Cs 250, a power supply Vdd 270 and an OLED 260. With use, the gate to source voltage (threshold voltage) of the “drive transistor” M2 may vary, thereby causing a change in the current passing through the OLED. This varying current contributes to the nonuniformity in the intensity of the display. Sometimes, in this scheme due to the production quality of the drive transistor M2240, it would lead to a consequence that ultimately produces threshold voltage variations in the pixels.
Another contribution to the nonuniformity in intensity of the display can be found in the manufacturing of the “drive transistor”. In some cases, the “drive transistor” is manufactured from a material that is difficult to ensure uniformity of the transistors such that variations exist from pixel to pixel.
Significant improvement in threshold voltage variations has been achieved in the prior art (See U.S. Pat. No. 6,229,506). In this patent, a design of four transistors two capacitors (4T2C) structure to compensate the threshold voltage of the drive transistor in each pixel was demonstrated to improve the uniformity in the intensity of the emitting pixels. However, in this scheme as shown in FIG. 3. The pixel structure adopts a data line 310, a scan line 320, a power supply Vdd line 305, an AZ line 390 and AZb control line 395, four transistors 330, 340, 370, 350, auto-zero capacitors 355, 380 and an OLED 360. In this scheme, the addition of the transistor is used to compensate the threshold voltage of the drive transistor M2340 in order to improve the uniformity of the emitting pixel. However, the addition of the device components occupy too much space in the tiny pixel structure and brings aperture ratio loss, moreover, there is always accompanying a contrast problem when conducting the auto-zero period, a slight current will run through organic electroluminescent (OLED) devices thus tends to reduce the contrast of the emitting pixel.
FIG. 4 depicts a schematic diagram of a time domain of the control signal in accordance with the FIG. 3, the time domain is separated as the auto-zero threshold voltage period and write data period. Before conducting the auto-zero period, M1330, M3370 are off, M2340 and M4350 are on, during this time period, the current running through OLED is the current of the previous frame time. Then after a while, M1330 is on, then M3370 is on sequentially, thus a connection of the drain and the gate of M2340 can be conducted as a diode. Then after a while, M4350 is off, then the current voltage of gate of M2340 raise to a value of Vdd-Vth (threshold voltage). At this instant, the M3370 is off, then the threshold voltage of M2340 will be recorded in the capacitor C2355, thus fulfill the auto-zero action.
It is a purpose of this invention to provide a new method to improve the uniformity of the emitting pixel and meanwhile to improve the aperture ratio of the organic electroluminescent (OLED) device.
It is another purpose of this invention to provide a new organic electroluminescent (OLED) device for display with improved contrast problem.